1. Field of the Invention
The present invention relates to a photomagnetic recording device and to a photomagnetic reproducing device, and more particularly to a device for improving the operation of a photomagnetic reproducing apparatus where a pit having a magnetic domain whose shape corresponds to recorded information is formed on a photomagnetic record medium.
2. Description of the Prior Art
In a photomagnetic recording apparatus as shown in FIG. 1, a record light beam 3 comprising a magnetizing magnetic flux 2 and a laser luminous flux irradiates a photomagnetic record disk 1 which rotates in the direction of arrow "a". An irradiation region on the disk formed by the record light beam 3 is heated to at least a prescribed temperature (Curie point temperature) whereby a magnetic domain constituting a pit 4 is formed in the irradiation region. Information is represented by timing (or record position) which forms the length of the pit 4 and/or the start end and the finishing end of the pit.
A light beam generator 5 for generating the record beam 3 as a function of a source signal S1 having the bit period T.sub.G shown in FIG. 2A functions to convert the source signal S1 into a code signal, for example, by NRZ conversion, producing a channel code signal S2 of bit period T.sub.c (=T.sub.G /2), as shown in FIG. 2B. When a bit of the channel code signal S2 is a logic "1", a record signal S3 whose logic level undergoes a transition at a time delayed by the delay time T.sub.R (=T.sub.c /2) form the beginning of the bit cell of signal S2 is generated as shown in FIG. 2C.
A light source comprising a laser diode for example is driven by the record signal S3, whereby the record light beam 3 emitted from the laser light source irradiates the photomagnetic record disk 1 with the timing of the transition in the record signal S3 rising to the logic "1" level. As a result, the pit 4 having a magnetic domain as shown in FIG. 2D is formed on the photomagnetic record disk 1.
The position at which the start end 4A of the pit 4 is formed corresponds to the bit cell containing a logic "1" bit in the channel code signal S2, and the position at which the finishing end 4B is formed corresponds to the bit cell containing the logic "1" bit that is subsequently generated in the signal S2. Thus the record information representing the channel code signal S2 can be recorded as a magnetization pattern on the photomagnetic record disk 1 with the length between or positions of the start end 4A and the finishing end 4B of the pit 4 identifying the occurrences of logic "1"s.
As shown in FIGS. 2E and 2F, a record signal S3 having a predetermined pulse width may be generated in synchronism with the timing of the channel code signal S2 as the channel code signal becomes a logic "1" so as to form a pit 4 having an isolated magnetic domain which may also be used.
However, the shape of the magnetic domain of the pit 4 formed on the photomagnetic record disk 1 is tapered towards the top or beginning of the start end 4A (having a so-called tear drop shape) in practice. Consequently, when the pit 4 is to be reproduced, there is the risk of generating a data error in the reproduced signal when the start end 4A is scanned.
In addition, since the width of the magnetic domain becomes narrower towards the top or beginning of the start end 4A of the pit 4, the leakage magnetic flux generated from the magnetic domain at this end becomes weak. As result, the precise start position of the start end 4A cannot be unambiguously reproduced.
The pit 4 exhibits the tear drop shape at the start end 4A because when the light beam 3 begins to irradiate the photomagnetic record disk 1, the disk travels in the direction "a", whereby the accumulated quantity of the laser light irradiated to the top of the start end is less than that in the rearward portion which follows the top end. It has been confirmed experimentally that the temperature distribution of the magnetic domain on the photomagnetic record disk 1 irradiated by the record light beam 3 becomes significantly larger into the tear drop shape in the case of a long irradiation time in comparison to the case of a short irradiation time as shown in FIGS. 3A and 3B.
FIG. 3B shows the shape of the magnetic domain when the record light beam 3 irradiates the disk continuously for a duration of 300 ns. In this case, the shape of an equi-temperature line is the tear drop shape and therefore the shape of the equi-temperature line representing the Curie point also has the tear drop shape. However, when the irradiation time of the record light beam 3 is reduced to about 1/3 of this duration, e.g. 100 ns, the shape of the equi-temperature line no longer appears as a pronounced tear drop shape.
As shown in FIG. 4, the degree of the asymmetry of the thermal distribution may become significant as the irradiation time duration becomes long.
When the record light beam irradiates photomagnetic record disk 1, the temperature rise produced at the position (x, y, Z) of the disk by thermal diffusion can be expressed by the following formula. ##EQU1## This is the convolution of the Green function G and the power distribution function Z, and it is seen that if the pulse width of the record signal S3 increases, the influence of the thermal diffusion becomes significant. As a result, the asymmetry of the thermal diffusion increases and the tear drop shape becomes significant.
To prevent this tear drop phenomenon of the magnetic domain, the intensity of the laser power of the record light beam forming the top end or beginning portion of the pit 4 may be controlled when forming the pit. In this case, however, bit separation during reproduction is not complete, and such intensity control is a less than satisfactory solution of the problem.